1. Field of the Invention
The present invention relates to the manufacture of hollow composite material bodies.
Although not exclusively, it is particularly appropriate to the manufacture of hollow three dimensional composite material bodies using a method in which, in a support mandrel whose outer surface corresponds to the inner surface of said hollow body, rigid members are lodged at least substantially orthogonal to said outer surface, then on the outer surface of said mandrel successive layers of superimposed and crossed fibers (glass, carbon, boron, etc.) are applied and the arrangement of said fibers is hardened with a hardenable binder, which forms a matrix and which may be introduced into said arrangement of fibers either by impregnating said fibers prior to application thereof on the mandrel, or preferably by impregnating said arrangement of fibers after formation thereof (for example by depositing said binder in the vapor phase) after which said mandrel is removed from the hollow body thus obtained.
The invention relates more particularly to a method for forming a mandrel for implementing the above described method of manufacturing hollow bodies, as well as a mandrel intended for manufacturing composite hollow bodies.
2. Description of the Prior Art
Such a method of manufacturing hollow bodies is for example described in the U.S. Pat. No. 3,577,294.
In this patent, said rigid members are themselves formed of fibers intended to remain imprisoned in said arrangement of layers so as to form one of the three dimensions of the hollow bodies. In order to be able to house such rigid members, the mandrel used is made on the surface from a material, such as a foam, into which said rigid members may readily penetrate first of all then be retained. Furthermore, in order to obtain hollow bodies with as precise a shape as possible, said hardenable binder is hardened when said arrangement of fibers is supported by said mandrel, thus ensuring stability of said arrangement during hardening. Thus, the mandrel is only removed after hardening of the fibers. For removing the mandrel from the hardened hollow body, the U.S. Pat. No. 3,577,294 provides for the mandrel used to be partially thermally destructible.
The fact that the arrangement of fibers must be hardened before removal of the mandrel is an important disadvantage. In fact, such hardening cannot be carried out at a high temperature for, if not, the foam material holding said rigid members and/or the thermally destructible part of said mandrel in position would be destroyed during hardening of said arrangement of fibers, so that the mandrel could no longer hold this arrangement in position. Now, the hardening operations may require the use of high temperatures (for example higher than 150.degree. C.) for polymerizing certain resins which may be used as hardenable binder. It is further known, as mentioned hereafter, that, in order to obtain high mechanical properties for the hollow body obtained in this way, it is often necessary to use densification operations requiring treatments at even higher temperatures.
In the French patent FR-A-2 587 375 a mandrel is described continuously withstanding a temperature of about 1650.degree. C. and which may temporarily withstand a temperature close to 2000.degree. C. This mandrel is for this purpose formed on the surface by a felt formed of silica fibers and a mineral binder formed for example of a mixture of silicates. The mandrel is obtained by bonding said silica felt on an internal support, for example made from cement or metal, and by curing said felt at 80.degree. C. for at least 48 hours in order to make it rigid through hardening of the mineral binder. The form of the mandrel is obtained, either by pressurized forming or molding of said felt (conceivably before curing), or by machining said felt after curing.
It will be noticed first of all that the structure of such a mandrel is heterogeneous (surface felt covering bonded to a support made from another material), so that the lack of homogeneity of said mandrel raises difficulties when it is subjected to high temperatures.
Furthermore, the temperature resistance performances put forward for such a silica felt mandrel appear justified, since silica melts at about 1600.degree. C. However, in this prior patent, it is stated, without any other particular information being given, that it is possible to form said mandrel from a felt made from a mineral material other than silica, such for example as carbon.
However, since carbon is consumed at about 400.degree. C., we must deduce therefrom that the temperature resistance performances of such a mandrel made from a carbon felt are very much less than those of the silica felt mandrel. Moreover, this prior patent does not mention a specific method of making the carbon felt rigid, so that we must deduce therefrom that such hardening is also obtained by means of a mineral agent of the type used for the silica felt mandrel, which further increases the lack of homogeneity of the mandrel.
However, it is interesting to have an homogeneous carbon felt mandrel for forming composite hollow bodies subjected to high temperature densification operations for carbon felt is cheaper than silica felt.
Thus, one of the objects of the present invention is to make it possible to obtain an homogeneous mandrel made from carbon felt.